A known exhaust purification system for an internal combustion engine includes a NOx storage-reduction catalyst (hereinafter simply referred to as a “NOx catalyst”) that stores nitrogen dioxides (NOx) in exhaust gas when the surrounding atmosphere is an oxidative atmosphere and that reduces the stored NOx when the surrounding atmosphere is a reduction atmosphere.
In the same manner as NOx, sulfur oxides (SOx) in the exhaust gas are also stored in the NOx catalyst. When the amount of SOx stored in the NOx catalyst increases, the NOx storage capacity of the NOx catalyst decreases. Therefore, in the exhaust purification system for an internal combustion engine including a NOx catalyst, what is referred to as SOx poisoning recovery control for reducing the SOx stored in the NOx catalyst is carried out.
Japanese Patent Application Publication No. JP-A-2005-90277 discloses a technology that starts SOx poisoning recovery control when an amount of SOx stored in a NOx catalyst reaches a maximum and subsequently stops the SOx poisoning recovery control when the amount of SOx stored in the NOx catalyst reaches a minimum. Japanese Patent Application Publication No. JP-A-2005-90277 discloses a technology that changes the maximum and minimum amounts for the stored SOx in accordance with the concentration of SOx in fuel that is used for operation of the internal combustion engine. Japanese Patent Application Publication No. JP-A-2004-108176 and Japanese Patent Application Publication No. JP-A-2005-76505 disclose technologies related to the SOx poisoning recovery control. Still further, Japanese Patent Application Publication No. JP-A-2003-206723 discloses a technology relating to a regeneration method for a particulate filter.